1. Field of the Invention
The invention relates to a HOM attenuated high frequency resonator provided with a cylindrical resonator cavity on the outer surface of which there are arranged three circular tapered waveguides with two symmetrically disposed connector elements, the cut-off frequency of the waveguide basic mode being kept constant over the length of the waveguides by varying the height of the connector element, the ridge waveguides at their smaller diameter end being provided with an impedance transformer for broadband HF adjustment of the coaxial line.
In electron storage rings for generating synchrotron radiation the brilliancy of the photon beams decisively depends upon the quality of the stored electron beam. Beam instabilities in particular negatively affect the generated brilliancy. The beam instabilities are cause by multibunch oscillations which cause an enlargement of the power width (longitudinal oscillations) and of the transverse emittance (transverse oscillations).
The multibunch oscillations are energized by the interaction of the electron packages with the higher order modes (HOM) of the acceleration resonator. By reducing the impedances of these HOM's below the specific critical impedance of every synchrotron source, the above-mentioned instabilities may be suppressed.
2. The Prior Art
For the suppression of the instabilities, various solutions in respect of resonators are known in the prior art.
For instance, Proc. Of the European Particle Accelerator Conference (EPAC 1990), Vol. 1, pp. 149; Proc. Of the European Particle accelerator Conference (EPAC 1996), Vol. 1, pp. 148 as well as Vol. 3, pp. 1976, describe the resonator at the Laboratori Nazionale di Frascati, INFN, at Frascati, Italy. This resonator consists of a bell-shaped resonator cavity provided with three long rectangular waveguides for HOM attenuation which are arranged at an angle of about 15 degrees relative to the axis of the resonator. As a consequence of this geometry and of the long steel pipes of truncated conical configuration for adjusting the large diameter at the pipe to the vacuum chamber of the ring, the installation length in the axial direction is about 2 m. The broadband rectangular waveguides are provided with an adaptor to a ⅞″ EIA coaxial line for coupling out the HOM energy. Not least because of the large diameters of the openings of the steel pipes is it possible to attain HOM impedances of relatively low value. This advantage is, however, achieved at the expense of a significantly reduced shunt energy of the basic made which results in higher, operating costs.
Another solution of coupling radially arranged waveguides to the interfering HOM's has been developed for the cavity (cavity resonator) by the Stanford Linear Accelerator Center, SLAC, at Stanford, U.S.A. (described, for instance, in SLAC-PUB-6129, LBL-30624, BECON-91, April 1991). This arrangement consists of a resonator cavity of spherical radial contour and three rectangular waveguides for the attenuation of HOM. The waveguides are arranged on the resonator at an angle of about 30 degrees relative to the beam axis, are then oriented parallel to the axis, and are finally, to save space, bent by 180 degrees. The HOM energy is absorbed in ferrite absorbers in the interior of the waveguides. As a consequence of the geometry of the arrangement the installation length in axial direction is about 1.8 m.
Both mentioned resonators have been developed for use in electron-positron-storage rings for high energy physics with extended straight sections and are thus of limited utility in connection with synchrotron radiation sources.
A resonator of the kind proposed by the Berliner Elektronenspeicherring-Gesellschaft fuer Synchrotronstrahlung m.b.H. is described in Proc. Of the European Particle accelerator Conference (EPAC 1996), Vol. 3, pp. 1940. In this case, the high frequency resonator is provided with a cylindrical resonator cavity on the outer surface of which there are arranged three circular waveguides for coupling to the HOM's each one of which is connected to a broad-band transition to a coaxial line (broad band circular waveguide to coaxial transition—CWCT). This arrangement allows reducing the necessary dimensions, in particular the installation length, relative to prior art devices. In Proc. Of the European Particle Accelerator Conference (EPAC 1998), Vol; 3, pp. 2065 a circular waveguide for such an arrangement has been described which is structured as a tapered ridge waveguide with a constant cut-off frequency and an impedance transformer to the ⅞″ coaxial line. As has already been mentioned, it was possible to reduce the dimensions of a HOM-attenuated high frequency resonator; however, the reduction of the shunt impedance of the fundamental mode is relatively large and the attenuation efficiency for higher order modes is insufficient.